(1) Field of the Invention
The present invention relates to the field of devices for end-to-end coupling between two rotary shafts, and it relates more particularly to such coupling devices provided with arrangements for reducing unbalance in a supercritical shaft that is driven in rotation.
The present invention provides such a coupling device, in particular in the context of end-to-end coupling between at least two rotary shafts for driving a rotorcraft rotor that is at a considerable distance from a drive source, and more specifically for driving a rotorcraft tail rotor.
(2) Description of Related Art
In the field of rotorcraft, at least one power plant serves to provide rotary drive for at least one rotor of the rotorcraft. The rotorcraft has a main rotor with a substantially vertical axis that provides it at least with lift, and usually also with propulsion, and indeed with guidance in flight. A rotorcraft frequently has at least one auxiliary rotor with an axis that is substantially horizontal, commonly a tail rotor or less frequently a propulsive propeller for a fast rotorcraft having long range.
Rotorcraft are frequently fitted with a main gearbox engaged with the power plant of the rotorcraft. Various rotors of the rotorcraft are driven in rotation from the main gearbox via transmission shafts. It is common for such transmission shafts to be very long axially, in order to rotate a rotor that is at a considerable distance from the main gearbox, such as a tail rotor.
This leads to the general problem of driving rotation of such remote rotors from the power plant by means of such a long transmission shaft. For example, for a tail rotor, the transmission shaft extends along a tail boom of the rotorcraft in order to drive the tail rotor from the main gearbox. The length over which the transmission shaft extends is potentially of the order of several meters.
The transmission shaft is commonly mounted on a carrier structure of the rotorcraft, and it is coupled to link shafts at its respective ends. An inlet link shaft engages with the drive source in order to drive rotation of the transmission shaft. An outlet link shaft driven by the transmission shaft is engaged with the remote rotor in order to drive it in rotation, in particular via a secondary gearbox.
Nevertheless, the environment of the transmission shaft is subjected to vibration under the effect of aerodynamic stresses to which the structure of the rotorcraft is subjected in flight. Such flight conditions make it difficult to support the transmission shaft all along its length by the carrier structure of the rotorcraft, in particular in a rotorcraft tail in the situation when the remote rotor that is to be driven is a tail rotor. In addition, the weight of the transmission shaft is not uniform along its entire length, thereby leading in particular to localized unbalance. Furthermore, at a threshold speed of rotation, the transmission shaft may potentially be set into resonance at a proper resonant frequency, thereby leading to structural vibrations of the transmission shaft.
In this context, it is conventional to distribute balancing rings along the transmission shaft and also support bearings and/or elastically deformable members for damping vibration.
It is desired to facilitate balanced rotation of the transmission shaft over its entire length and to protect it as well as possible from the various kinds of vibration to which it is subjected. Nevertheless, such solutions involve the use of a large number of parts for mounting the transmission shaft on the rotorcraft. This leads to considerable manufacturing and maintenance costs for the rotorcraft, and it is desirable to reduce such costs. In addition, such a large number of parts increases the weight of the rotorcraft, which is always to be avoided in the field of aviation.
An advantageous solution consists in using a supercritical flexible shaft to form the transmission shaft. Using a supercritical transmission shaft makes it possible in particular to avoid mounting the transmission shaft to the structure of the rotorcraft via the various bearings and damper members that are conventionally distributed along the length of the transmission shaft.
Nevertheless, the use of a supercritical transmission shaft involves accommodating its deformation between nodes that are in alignment on the axis along which it naturally extends when at rest, and more particularly to accommodate its deformation in angular deflection between its ends that are engaged with the link shafts.
There then arise difficulties in achieving end-to-end coupling between the supercritical transmission shaft and the link shafts. Such coupling needs to procure well-balanced rotation of the transmission shaft. Optimized transmission of torque between the transmission shaft and the link shafts needs to be encouraged. Account must also be taken not only of the angular deflection of the supercritical transmission shaft relative to the axes of the link shaft, but also of potential angular misalignment, radial misalignment, and/or axial offset between the shafts.
In order to procure good torque transmission, the means for transmitting torque between the transmission shaft and the link shafts are conventionally robust. Such robust torque transmission means are commonly of the type making use of fluting, notched or toothed members, or other analogous means for robust radial engagement.
In the context of a supercritical transmission shaft, flexible junction means are provided between the supercritical junction shaft and the link shafts. Such a flexible junction makes it possible, in spite of the supercritical transmission shaft deforming between its ends, to avoid seizing or even jamming in operation of the robust torque transmission means that are conventionally used. For example, a flexible end-to-end coupling member is interposed between the transmission shaft and a link shaft. Such a flexible member accommodates movement in angular deflection of the transmission shaft relative to the link shafts. By way of example, such a flexible coupling member is arranged as a plate made up of juxtaposed strips, said plate being placed in radially opposite engagement respectively with the transmission shaft and with a link shaft.
The present invention takes into consideration more specifically the management of angular deflections of the transmission shaft relative to the axes along which the link shafts extend. A flexible connection between the transmission shaft and the link shafts makes such angular deflection possible, but it is nevertheless desirable for it to be damped. A supercritical transmission shaft is commonly supported at its end by respective rolling bearings carried by a carrier structure, and means for accommodating and damping movement in angular deflection of the supercritical transmission shaft are interposed between the carrier structure and the transmission shaft.
In general, the solutions used in the field of supercritical transmission shafts make use of means for damping the bending deflection of the transmission shaft. By way of example, such damping means make use of hinged structures distributed along the transmission shaft and capable of damping the bending deflection of the transmission shaft relative to the axes of rotation of the link shafts. Although such hinged structures present the advantage of being inexpensive and light in weight, they nevertheless present the drawbacks of being subjected to rapid aging and of it being necessary to install a plurality of them along the transmission shaft, thereby leading to unwanted extra weight in the rotorcraft.
Consequently, it is necessary to provide arrangements for installing and operating said damper means that tend to provide long-lasting reliability and accurate damping of the deformation of the supercritical transmission shaft, while avoiding making the rotorcraft heavier.
More particularly, it should be considered that the arrangements for mounting and damping deformation of a supercritical transmission shaft, and in particular such deformation in angular deflection, tend to make the overall structure of the end-to-end coupling device between the transmission shaft and the link shafts more complex and excessively heavy. Such complex and heavy structures should be avoided, in particular in the field of aviation, and they are not simple to maintain.
It is also necessary to take care that the provisions for mounting and damping angular deflections of the supercritical transmission shaft do not shorten the lifetime of structural members of the coupling device, while avoiding making that structure excessively complex. The arrangements of mounting and damping angular deflections of the supercritical transmission shaft must have no effect on the structure or on the specific operation of the members used together for coupling the transmission shaft with the link shafts, for damping the angular deflections of the transmission shaft, or indeed for guiding and procuring balanced rotation of the transmission shaft relative to the carrier structure.
For further knowledge of a technological environment close to that of the present invention, reference may be made to the following documents: U.S. Pat. No. 3,425,239 (Boeing Co.); FR 2 962 176 (Turbomeca); EP 0 722 544 (Minnesota Mining & Mfg); and EP 2 367 268 (Bell Helicopter Textron Inc.).